Clinical samples from patients with acute or chronic non-A, B, C, D, E hepatitis in the United States are being studied for biological, serological or molecular evidence of transmissible agents. Patients with fulminant non-A, non-B hepatitis remain a diagnostic enigma and may be infected with one or more previously unrecognized viruses. We are attempting to discover the etiology of this disease. Evidence for the existence of an additional water-borne hepatitis virus has come from sero-epidemiologic studies in India and Saudi Arabia. We are attempting to transmit an agent from clinical specimens of such patients. Some years ago, a hepatitis virus was reported to be transmissible from a non-A, non-B hepatitis patient to marmoset (tamarin) monkeys. The agent, called the GB agent, could be serially transmitted in marmosets and was partially characterized. Recently the GB agent was cloned and sequenced and shown to be two separate viruses (GBV-A and GBV-B), each distantly related to hepatitis C virus as well as to a previously unrecognized human virus. The GB agents and the newly discovered human virus are being studied in primates and in vitro. An infectious cDNA of GBV-B has been constructed. To date, the newly recognized human virus (HGV/GBV-C) has been transmitted to chimpanzees and the resulting infection characterized. In addition, several new viruses have been discovered in New World monkeys. These are being further characterized. Serologic evidence of infection of swine with hepatitis E virus (HEV) was obtained. A new and unique HEV strain was recovered from infected swine and characterized. Similar serologic evidence for infection of wild rats with HEV has also been obtained and the infecting agent is being sought. Modern techniques of molecular biology have been used to discover new viruses in recent years. These are now being applied to prospectively collected sera from patients with transfusion-associated hepatitis in a search for new hepatitis viruses that may cause up to 15% of such hepatitis in the US. One such agent is the TT virus (TTV), a unique DNA virus recovered in Japan from transfused patients. The virus is ubiquitous and genetically highly heterogeneous. The HVS is exploring its role in acute and chronic hepatitis following exposure to blood. In addition, unique strains of TTV have been recovered from chimpanzees. The implications of the high infection rate of humans and chimpanzees with TTV are being examined. Kawasaki Disease is a life-threatening illness of young children. It has the epidemiologic characteristics of an infectious disease. The HVS is attempting to transmit a putative agent from acute phase clinical samples of children with Kawasaki Disease to nonhuman primates. Should transmission be successful, the transmissible agent will be characterized.